Undostack: Fix segfault when trying to fetch macro on -1 index

Currently, if user adds a command to the undostack, and then removes it,
and then adds another command - this causes a segfault, because the
current undo position pointer of undostack is being moved to -1. This
patch removes a possibility of this segfault by clearing macros and
commands vector. If user has undo'd all commands (their position is at
-1), then it means that whenever eraseRedundantCmds() is called, a
macro or command has been pushed. If that happened, then we clear every
command that's currently in "undo" state. In this case everything to the
right is in undo state, so we simply need to clear everything.

source/undostack/undostack.cpp

@@ -184,6 +184,13 @@ void UndoStack::addMacro(UndoMacroFactory &macroFactory)
  */
 void UndoStack::eraseRedundantCmds()
 {
+    // If user has undo'd all the commands, then erase everything
+    if (m_undoPos < 0) {
+        m_cmds.clear();
+        m_macros.clear();
+        return;
+    }
+
     // Erase any command that was set to obsolete
     std::erase_if(m_cmds, [](const auto &cmd) { return cmd->isObsolete(); });